Compact microwave spectrometer

ABSTRACT

The development of a simple and inexpensive microwave spectrometer for the study of spin resonance phenomena has been made possible by the inclusion of a solid state microwave energy source within the same resonant cavity as the sample to be investigated. A hypersensitive condition of oscillation of the microwave source has been found which leads to a spectrometer rivaling the sensitivity of much more complex and expensive conventional apparatus. Development models constructed using Gunn type microwave oscillator diodes show that, in addition to the use of conventional microwave detection, the output signal can be detected by observation of the diode bias current, obviating the use of a separate microwave detector.

ilaited States Patent Rupp, Jr. et al.

[451 Sept. 12, 1972 I COMPACT MMIROWAVE SPECTROMETER [73] Assignee: BellTelephone Laboratories, Incorporated, Murray Hill, Berkeley Heights, NJ.

[22] Filed: April 28, 1970 [21] Appl. No.: 32,538

[52] US. Cl. ..324/0.5 R, 331/107 G [51] Int. Cl. ..G01n 27/78 [58]Field of Search ..324/0.5 A, 0.5 AC, 0.5 AI-I;

[56] Reierences Cited UNITED STATES PATENTS 3,582,778 6/1971 Faulkner..324/0.5 OTHER PUBLICATIONS L. W. Rupp, W. M. Walsh & A. Steinfeld-Simplified Microwave Frequency Electron Spin Resonance Spectrameter, Am.Journal of Physics, 38(2), Feb. 1970 pp.238- 242.

Primary Examiner-Michael J. Lynch Attorney-R. J. Guenther and Edwin B.Cave [57] ABSTRACT The development of a simple and inexpensive microwavespectrometer for the study of spin resonance phenomena has been madepossible by the inclusion of a solid state microwave energy sourcewithin the same resonant cavity as the sample to be investigated. Ahypersensitive condition of oscillation of the microwave source has beenfound which leads to a spectrometer rivaling the sensitivity of muchmore complex and expensive conventional apparatus. Development modelsconstructed using Gunn type microwave oscillator diodes show that, inaddition to the use of conventional microwave detection, the outputsignal can be detected by observation of the diode bias current,obviating the use of a separate microwave detector.

2 Claims, 1 Drawing Figure MODULATION COMPACT MICROWAVE SPECTROMETERBACKGROUND OF THE INVENTION 1. Field of the Invention The invention liesin the field of. apparatus for measurement of the microwave absorptionspectra of matter.

2. Description of the Prior Art The microwave absorption of matter in amagnetic field is used extensively as a diagnostic tool by chemical,physical and biological investigators. A sample of the matter to bestudied is placed in a d.c. magnetic field and irradiated with amicrowave electromagnetic field. As the d.c. magnetic field is slowlyvaried, resonant absorption of the microwave energy is observed which isdirectly related to the electronic structure of the sample. From this,information about the sample can be derived. The simplest form ofapparatus required for such a measurement includes a source of microwaveenergy such as a klystron, a length of waveguide within which the sampleis fixed and a microwave detector. More sensitivity can be realized ifthe portion of waveguide within which the sample is located is made intoa cavity by, for instance, the introduction of transverse walls withsmall coupling irises. However, this increased sensitivity brings alongwith it electronic complexity. Since two cavities are now involved, thecavity within the klystron and the sample cavity, an AFC loop betweenthe microwave detector and the microwave source must be introduced toprevent the drift of the source frequency relative to the resonantfrequency of the cavity containing the sample. Such drifts orfluctations would yield spurious signals. Instruments of this sortcosting many thousands of dollars are now being marketed by instrumentmanufacturers for research use.

SUMMARY OF THE INVENTION It has been found that the spectrometer can begreatly simplified if the microwave source and the sample are placed inthe same cavity. If this is done the frequency of the generatedmicrowave energy cannot drift relative to the resonant frequency of thesample cavity, since the generated frequency is defined by this samecavity. This allows the elimination of the AFC loop and allows theconstruction of a simple compact and inexpensive spectrometer. Sincethis requires that the source be in the magnetic field supplied to thesample, a source whose frequency does not change appreciably withchanging magnetic fields is required. It was found that semiconductingmicrowave oscillator diodes meet this requirement. In the low to mediumranges of the microwave spectrum high field domain (Gunn) type devicesare suitable. At higher frequencies, into the millimeter range, LSA,IMPATT or tunnel devices may be used.

Greatly increased sensitivity has been observed in some experimentalmodels when the diode voltage is reduced below that voltage producingmaximum power output. This hypersensitive operation leads tospectrometer sensitivity rivaling that of much more expensive apparatus.The attainment of this hypersensitive operation depends upon thecharacteristic of the particular device used and on the electricalloading.

Under favorable circumstances an output signal can be derived directlyfrom the observation of the source bias current, thus eliminating theneed for a separate microwave detector. Microwave spectrometers builtalong the lines described above may be used for production line testing(e.g., detection of impurities) or as a pedagogical tool inundergraduate and graduate college laboratories as well as forscientific research.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematicrepresentation of a microwave spectrometer of the disclosed type withancillary instrumentation.

DETAILED DESCRIPTION OF THE INVENTION Basic Spectrometer The drawing isa schematic representation of a basic spectrometer of the type disclosedhere. The sample cavity 1 is a length of waveguide which, in theexperimental model constructed, was rectangular in cross section.Situated within the cavity is the sample to be measured 2. The sample 2may be placed within the cavity through an entrance port provided 3 orother insertion means so arranged as not to require the disassembly ofthe spectrometer. Also placed within the cavity is the microwaveoscillator diode 4 which serves as the source of the microwaveelectromagnetic field. In the low to medium ranges of the microwavespectrum, high field domain (Gunn) type devices are most suitable. Athigher frequencies, into the millimeter range LSA, IMPATT or tunneldevices may be used. This source 4 is energized by a dc. current fromthe bias supply 5. This bias supply may be provided with a currentmeasuring device 6, which may be simply a series resistor. One end ofthe cavity is formed by a transverse conducting wall 22 and the otherend by a sliding short 8 which is used to tune the cavity. The cavitymay also be provided with other tuning devices such as a tuning screw,which may be a metal screw protruding through the wall of the cavity.This screw may be employed to adjust the frequency of the cavity ordistort the field pattern within the cavity in order to achieve optimumcoupling to the sample. Once the required cavity length has beendetermined either theoretically or experimentally, the sliding short 8may be eliminated and a fixed short circuit wall can be substituted or,in some cases, the cavity can be terminated in an open circuit by merelycutting the waveguide to the appropriate length.

A d.c. magnetic field is applied to the sample by a magnet whose polepieces 9 are illustrated. In order to observe the resonant absorptionthe magnitude of the magnetic field is varied by small amounts relativeto the large d.c. field by means of the modulating coils 10. Themodulating coils are supplied by the modulation source 11 which also maysupply an output 12 to the X- axis input 20 of a display device 19 suchas an oscilloscope. In order to observe the magnitude of the microwavefield, a microwave detector 15 can be used. In this case a small irisopening 13 is supplied in the conducting transverse wall 22 and thedetector diode 15 is placed within a cavity 14 with its own slidingshort 23. This sliding short is used to optimize the coupling of themicrowave field to the detector 15. The detector cavity 14 can also besupplied with a frequency measuring device such as a cavity-type wavemeter 17. If a cavity-type wave meter is used, the detector cavity canbe made to exhibit the frequency changes which occur during the resonantabsorption of the sample by adjusting the wave meter 17 to the pointwhere the meter 17 has a large change of absorption with frequency. Thedetected signal from the diode 15 can be lead to the Y- input 21 of thedisplay device 19. The display device can be provided with a suitableelectronic amplifier in order to achieve the required level ofsensitivity. Greatest sensitivity usually requires the use of a lock-indetector. It has been found in some cases, usually for the moreefficient oscillator diodes 4 that the absorption of the sample 2 can beobserved in the current which is drawn by the diode 4 from the biassupply 5. In such cases an output 7 can be taken from the currentmeasuring device 6 and lead to the Y-input 21 of the display device 19.This eliminates the need for the microwave detector 15 and its cavity14.

Hypersensitive Operation During work with experimental models such asillustrated above, it was found that the microwave oscillations could bemade to exhibit a hysteresis effect. That is as the voltage on the biassupply 5 is increased from zero, no microwave oscillation is observeduntil the upward breakover point typically in the neighborhood of 8 tovolts in the devices used, is reached. As the voltage increases further,the power output of the diode 4 reaches a maximum and then starts todecrease. If the voltage is then slowly decreased oscillation ismaintained to voltages less than the upward breakover point until thedownward breakover point is reached, typically between 6 and 7 volts inthe devices used. It was observed that in the region of oscillationbetween the upward breakover point and the downward breakover point, thesensitivity of the spectrometer shows a marked increase. This region ofoperation is referred to here as the hypersensitive region. Theattainment of this hypersensitive operation depends primarily on thediode used and on the electromagnetic loading of the cavity. It isbelieved that relatively lightly doped Gunn and LSA devices arepreferable for this hypersensitive operation (doping .1: length) lessthan -10 cm" Sensitivity A standard measure of the sensitivity ofmicrowave spectrometers is the minimum number of paramagnetic electronspins which can be observed with a one-to-one signal-to-noise-ratio, ina measuring device with a one cycle per second bandwidth, if the widthof the absorption line is one oersted. According to this measure themost sensitive high quality experimental devices presently available candetect as few as 10 spins. The spectrometer disclosed here, when biasedfor optimum power output, can detect as few as 10 spins. However, whenoperated in the hypersensitive region, the sensitivity is increased byan order of magnitude and as few as 10 spins can be observed. This levelof sensitivity is quite useful for all but the most demanding researchexperiments.

The spectrometer presented in the drawing is intended to be merelyillustrative of the various elements which may be employed in anyparticular apparatus design. In addition to the variations describedabove,

he ossibl variation of ca i esi n a cro section imoevn to the art aremany fo a buta'ou l d be siinple extensions of the basic ideas disclosedhere.

What is Claimed is:

1. Method for the investigation of the properties of matter bymeasurement of the resonant interaction between electromagnetic fieldsof frequencies greater than 3 x 10 cycles per second and a portion ofthe matter comprising:

a. placing the portion in a resonant cavity, which cavity includes asemiconductor microwave oscillator device as a source of theelectromagnetic field;

. applying a variable magnetic field to the portion;

0. applying a bias voltage to the semiconductor microwave oscillatordevice;

. adjusting the bias voltage of said oscillator device to a level atleast 5 percent below the level of said bias voltage required toinitiate the production of said electromagnetic energy so that saidsource of electromagnetic energy is oscillating in a hypersensitivestate; and

e. observing an output signal dependent upon the resonant interactionbetween the portion and the electromagnetic field.

2. Apparatus for the investigation of the properties of matter bymeasurement of the resonant interaction between electromagnetic fieldsof frequencies greater than 3 x 10 cycles per second and a portion ofthe matter subjected to a varying magnetic field, said apparatuscomprising:

a. a resonant cavity for the insertion of the portion;

b. a semiconductor microwave oscillator device positioned in said cavityas a source of the electromagnetic field;

c. insertion means required to insert the portion within the resonantcavity at a position which is favorable for the interaction of theportion with the electromagnetic field; and resonant signal output meansincluding means for the measurement of the variation of the bias currentthrough the semiconductor microwave oscillator device as correlated withthe variation of the magnetic field.

1. Method for the investigation of the properties of matter bymeasurement of the resonant interaction between electromagnetic fieldsof frequencies greater than 3 x 108 cycles per second and a portion ofthe matter comprising: a. placing the portion in a resonant cavity,which cavity includes a semiconductor microwave oscillator device as asource of the electromagnetic field; b. applying a variable magneticfield to the portion; c. applying a bias voltage to the semiconductormicrowave oscillator device; d. adjusting the bias voltage of saidoscillator device to a level at least 5 percent below the level of saidbias voltage required to initiate the production of said electromagneticenergy so that said source of electromagnetic energy is oscillating in ahypersensitive state; and e. observing an output signal dependent uponthe resonant interaction between the portion and the electromagneticfield.
 2. Apparatus for the investigation of the properties of matter bymeasurement of the resonant interaction between electromagnetic fieldsof frequencies greater than 3 x 108 cycles per second and a portion ofthe matter subjected to a varying magnetic field, said apparatuscomprising: a. a resonant cavity for the insertion of the portion; b. asemiconductor microwave oscillator device positioned in said cavity as asource of the electromagnetic field; c. insertion means required toinsert the portion within the resonant cavity at a position which isfavorable for the interaction of the portion with the electromagneticfield; and d. resonant signal output means including means for themeasurement of the variation of the bias current through thesemiconductor microwave oscillator device as correlated with thevariation of the magnetic field.